This invention relates to the creation of volumetric data sets from multiple two-dimensional analog images. Embodiments of the invention provide both methods for assembling a number of two-dimensional images into a volumetric data set and automated systems for performing such methods. The invention has particular application in the generation of volumetric data sets representing the spatial distribution of absorbed radiation dose for radiation exposures to be used in conformal radiosurgery.
There are various situations in which it is necessary to obtain a volumetric data set which represents the spatial distribution of some physical quantity. In many such cases there is no practical cost effective way to directly acquire data for the volumetric data set. Radiosurgery is one such situation.
Radiosurgery uses radiation beams to treat tumours. It is desirable to provide dose distributions which provide high doses within a tumour being treated and low doses everywhere else. Conventional linear accelerator-based stereotactic radiosurgery employs a number of circular photon beams or beam arcs to create sharply defined, but approximately spherical or ellipsoidal dose distributions.
The inherent symmetry of dose distributions produced by circular beams permits the actual dose distribution to be measured by taking 1 or 2-dimensional profiles using standard dosimetry techniques.
Many tumours, however are not spherical or ellipsoidal. It is desirable to provide shaped dose distributions to treat such tumours. Technology for the delivery of irregularly-shaped dose distributions has evolved rapidly. Radiosurgery systems which employ technologies such as fixed conformal collimation, dynamic conformal collimation, micro-multileaf collimation and intensity modulated radiation therapy are able to create non-spherical volumetric dose distributions suitable for treating non-spherical lesions. A dose distribution may be provided by way of a planned series of exposures to radiation. Various dose calculation algorithms may be used to mathematically model the dose which will be provided by a given configuration of a radiosurgery system. While the sophistication and accuracy of dose calculation algorithms has increased, it is necessary to be able to measure the actual volumetric dose distribution provided by a radiosurgery system apparatus so as to ensure that the apparatus is functioning properly and is producing the predicted dose distribution. Radiosurgery systems should provide the predicted dose distribution within a spatial accuracy of about xc2x11 mm or better and a numerical accuracy of about xc2x15%, preferably xc2x12%, or better throughout the volume of the dose distribution. Measuring such distributions requires a dosimeter capable of measuring integrated dose in three-dimensions.
Traditional dosimeters, such as miniature ion chambers, thermoluminescent dosimeters, diodes and diamond detectors have been used for radiosurgical dosimetry but measure doses only at single points or along one-dimensional lines (U.S. Pat. No. 5,635,709 describes an example of a system which senses radiation intensity at a point). Ferrous sulfate and BANG gel dosimetry techniques (for example, see U.S. Pat. No. 5,633,584 which describes a system for 3-dimensional dosimetry which uses a polymerizable gel) provide 3D maps of administered dose. However, both of these methods remain expensive because an NMR scanner is required for dose calibration and readout. The gels also require special handling. Further, the use of NMR to read out dose data imposes significant limitations on BANG or Fricke gel dosimetry in terms of achievable spatial resolution and signal-to-noise ratio.
Radiographic and radiochromic films are inherently planar dosimeters. Such films can be placed in the path of a photon beam to provide a two-dimensional dose profile. The dose profile may be obtained by scanning the film using a conventional densitometers or CCD-based digitizer. Radiographic and radiochromic films provide the advantages of sub-millimeter spatial resolution and high signal-to-noise ratio. Such films are affordable and accessible.
Radiographic films typically have silver halide emulsions which are exposed by interaction with photons. After exposure the films are developed. For larger (i.e.  greater than 10 cmxc3x9710 cm) conventional radio-therapy photon fields, however, the accuracy of radiographic film dosimetry has been limited by the over response of silver halide film emulsions to low energy scattered photons. Below energies of approximately 400 keV, the mass attenuation coefficient of typical emulsions diverges rapidly from that of tissue. Therefore, changes in the photon population in this low-energy region cause variations in emulsion sensitivity. This effect produces a systematic shift in the optical density-to-dose sensitometric calibration curve with depth thus necessitating corrections to the measured optical density distributions.
Fortunately, small (less than about 40 mm in diameter) high energy, for example 6 MeV, radiosurgical photon beams exhibit spectra composed predominantly of primary photons. The inventors have conducted Monte Carlo simulations which have demonstrated that, unlike large radiotherapy photon fields, in small radiosurgical beams increases in the spectral component below 400 keV are negligible to depths of 20 cm. Typically 5% or less of the incident spectrum in a 6 MeV radiosurgical photon beam exists in this low-energy region. The majority of the spectral population is found in the range where the effect of emulsion inhomogeneity is minimized even at depth. The effect of this high primary-to-scatter ratio is manifest in measured sensitometric curves, which are invariant (to within 1.5%) with depth (to 20 cm in a phantom) and with field size (within the range of field diameters used in radiosurgery).
Radiochromic films have one or more thin microcrystalline layers of monomer which polymerize in response to irradiation. Currently available radiochromic films turn blue following exposure to radiation and require no chemical processing. The constituents of the film are essentially tissue equivalent.
There is a need for methods and systems for relatively quickly and accurately measuring the dose distributions provided by radiosurgery systems and, more generally, three-dimensional distributions provided by other systems.
This invention provides a method for creating a volumetric data set containing data representing a three-dimensional distribution of a physical quantity. The quantity may be, for example, the integrated dose produced within a volume by one or more radiation sources. The radiation sources may be sources in a radiosurgery system. The method comprises providing a plurality of spaced apart two dimensional analog sensors in fixed orientations; simultaneously creating a plurality of two dimensional analog images by exposing the plurality of sensors to the distribution; scanning the two dimensional analog images to yield digitized images; and, before scanning the analog images providing fiducial marks on each of the plurality of sensors, the fiducial marks identifying an order and sequence of the sensors. The sensors may be film. In specific embodiments of the invention the sensors are parallel, spaced apart sheets of X-ray film.
Preferably the sensors are generally planar so that each sensor measures a cross-section of the distribution. Each sensor is preferably parallel to and spaced apart from adjoining sensors. The fiducial marks are useful for automatically identifying the orientation and sequence of films within a set of films. The fiducial marks may be provided by exposing selected locations on the films, or other sensors to radiation, which may be light.
the digitized images are delivered to a programmed computer. The programmed computer locates the fiducial marks, and, from the fiducial marks, identifies a sequence of the digitized images. The method preferably includes determining from the fiducial mark whether an image of a sensor is correctly oriented and, if the image is not correctly oriented, applying one or more rotation or flip transformations until the image is correctly oriented. This causes the method to be insensitive to human error which might result in one or more images being placed out of sequence or in the wrong orientation. The fiducial mark on each of the sensors is preferably located along one edge of the sensor and, when the sensors are in the fixed orientations, all of the fiducial marks are located between a midpoint of one edge of the sensor and a corresponding corner of the sensor. Where the distribution is a dose distribution produced by a radiosurgery system, the method may include exporting a volumetric data set in a format which comprises landmarks which specify points in the volumetric data set corresponding to known locations in a coordinate system of the radiosurgery system. This facilitates coregistering the volumetric data set with an intended dose distribution.
Another aspect of the invention provides a computer system for creating a volumetric data set from a plurality of two-dimensional digitized images. The computer system comprises a processor and stored instructions which, when run on the processor, cause the processor to: for each of a plurality of two-dimensional digitized images locate a fiducial mark on the image; determine from a location of the fiducial mark whether the image is correctly oriented; if the image is not correctly oriented, apply one or more inversions and rotations to the image until it is correctly oriented; determine from a location of the fiducial mark the sequence of the image relative to the other images; and, populate a three dimensional array with data from the digitized images. Preferably the software further causes the processor to compute predicted values between points in the three dimensional array by interpolation.
Further features and advantages of the invention are described below.